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  1. #1

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    how accurate are gps numbers

    how accurate are gps numbers. i went on and took some speed runs with my ski and the numbers that came back where a little on the high side.


    its a garmin etrex


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    DAGO RACING CREW 97GPSLEEPER's Avatar
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    The are very accurate. One cause of a high reading would be if it was bouncing around in the glove box. At Mudbug my GPS was reading a few tenths slower than the radar gun. What were your numbers? Maybe your ski just runs really good!

  3. #3
    The ski's have taken a "backseat" to the Corvette DarthAWM's Avatar
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    Quote Originally Posted by stingray152003 View Post
    how accurate are gps numbers. i went on and took some speed runs with my ski and the numbers that came back where a little on the high side.


    its a garmin etrex
    Depends on the satellite reception at the time you are running, but they are generally considered within a 1/10th.

    Now you do know about the GPS hiccup? This is when an increadibly high max speed number occurs because of losing or gaining a satellite in the sky.

  4. #4
    Duke's Avatar
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    I'm sure it has to do with reception like Darth says but here in Fl they seem pretty accurate. We have even put several different ones on the same boat with exactly the same max speeds every time.

    The reason the radar gun usually shows a little higher is due to its refresh rate. Typically they refresh every 1/100 of a second verses every 1/2 second on most GPSs. This will pick up all the peaks much better.

  5. #5

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    okay i had it Velcroed to the handle bars. i had 5 satellites took a while to get them but once it did they held a high signal and i had a 6th that kept bouncing on and off.

    02 gp800r 59.6 going up river no engine mods just few hull mods. seems way to high to me i was told by a few people that stock they do 51.

    the average most were 58.5-59.1. they seem very high.

    i have a hard time trusting the those numbers
    Last edited by stingray152003; 06-28-2008 at 11:37 AM.

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    Duke's Avatar
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    Quote Originally Posted by stingray152003 View Post
    okay i had it Velcroed to the handle bars. i had 5 satellites took a while to get them but once it did they held a high signal and i had a 6th that kept bouncing on and off.

    02 gp800r 59.6 going up river no engine mods just few hull mods. seems way to high to me i was told by a few people that stock they do 51.

    the average most were 58.5-59.1. they seem very high.

    i have a hard time trusting the those numbers
    If you had several different readings all close to the same then thats your speed. Have no idea what speeds the 800 is supposed to go but looks like your close to 60.

  7. #7
    GREENSMACKER's Avatar
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    Quote Originally Posted by stingray152003 View Post
    okay i had it Velcroed to the handle bars. i had 5 satellites took a while to get them but once it did they held a high signal and i had a 6th that kept bouncing on and off.

    02 gp800r 59.6 going up river no engine mods just few hull mods. seems way to high to me i was told by a few people that stock they do 51.

    the average most were 58.5-59.1. they seem very high.

    i have a hard time trusting the those numbers
    my friend had a 800 gps was 58 all day long no mods added a rideplate got 59.? cant remember

  8. #8
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    GPS speed is very accurate. What you have to take into consideration is that you are on a moving surface-water. Whether its a river or a lake, its moving, or has a current. Say for example your making a speed run on a river that is flowing at 2mph. Your speed over the earths surface-thats what the GPS is measuring, will be 2mph faster down river, and 2mph slower up river.
    I ride on Lake Havasu, which is basically a wide spot between dams(Parker to the south, Davis to the north) on the Colorado river. Where I do my speed runs(area 51), the current is approximately 1mph. What I do is make one run up river, then turn around and wait for the water to settle, and make another run down river, in the exact same area. I then average those speeds and post my numbers.

    Even if your on a lake, its best to use the same technique, one run in opposite directions, then take the average. The GPS is measuring ground speed-your speed over the earths surface. The refresh rate will vary with different GPS'. And you only need 3 satellites to achieve accuracy. If you have 5 or 6, that's great, but the difference will be less than 1/10 of a mile per hour. Generally, 3 satellites gives you aproximately 60ft of accuracy; i.e. 60ft is still less than 1/10 of a mile, which is 520ft. If you have 6 satellites, you accuracy is about 20 ft. If your GPS has WAAS, then your accuracy is close to 8ft. WAAS is primarily used in aircraft navigation. I use a WAAS enable GPS in the helicopter I fly, and it has a DOP of about 5 to 8 feet.

    The military GPS units are accurate to within a few inches.....WAAS is a radio signal that is ground based. It supplies a correction signal to the GPS unit in your aircraft, car, or boat-if the unit is equipped for this function, that allows for a more accurate reading of your position.

    Sorry for the long explanation-hope it helps.

    Radar is slightly more accurate than a gps because it updates much more rapidly. However, it has the same faults as the GPS. Your on a moving surface that has current, and the radar operator is stationary on the ground. The difference between GPS speed, and radar speed, shouldn't vary by more than a tenth of a mph. But that takes into consideration that you ski doesn't jump 1/10th a mph for a millisecond to two, which the radar would catch, but the gps might not catch. Its all just splitting hair though. I just average my speed and round down slightly so that I can be as honest as possible.

  9. #9

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    thanks guys very insightful. you guys gave me all the info i needed. thank you every one

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